<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    
  </style>
</head>
<body>
  
</body>
<script>

  var a;
  console.log( a );
  a = 10;
  console.log( a );

  console.log( a );
  
  console.log( "a" );
  
  console.log( a );


  var b = a;
  console.log( b );

  var c,d=20,e,f=30;

  console.log(c);
  console.log(d);
  console.log(e);
  console.log(f);

  var g = null;
  console.log(g);

  



</script>
</html>